Headwater Valley Research Articles

Eucalyptus Planting in Pasture Matrix and Erosive Reactivation of Gullies in the Sesmaria River Basin (SP/RJ): a cycle of instability in headwater valleys

In the 21st century, eucalyptus plantations have expanded in the middle valley of the Paraíba do Sul River, in the southwestern region of Brazil, replacing degraded pastures. Some studies associate these plantations with the reduction of groundwater levels, but field observations have indicated that erosive activity related to permanent and temporary underground water flows is more significant. This study focuses on the reactivation of erosive processes leading to the regressive growth of gullies after the introduction of eucalyptus in pasture areas. Infiltration experiments, soil suction monitoring, well water levels, and gully erosion were conducted after the introduction of eucalyptus. The results indicated that eucalyptus cover facilitated rainwater infiltration, feeding temporary and suspended aquifers. These flows, combined with the permanent aquifer, caused erosion due to excessive pressure on gully margins, bottoms, and slopes. Erosion rates decreased over the years after the introduction of eucalyptus. Tree vegetation began to recover in the gullies, indicating a trend towards stabilization. In response to changes in land use, the drainage and erosion system are undergoing an internal adjustment period in search of a new balance and stability.

Macroinvertebrate assemblages from a stream-wetland complex: a case study with implications for assessing restored hydrologic functions

Legacies of past land use persist today in the form of incised, single-threaded stream channels with dramatically different hydrologic functions of pre-colonial stream valleys. Restoration practices that aim to return lost hydrologic functions by re-establishing floodplain and groundwater connections should result in stream habitat and biological assemblages that differ from modern, single-threaded channels. The aim of this case study was to identify attributes of macroinvertebrate assemblages that might serve as biological indicators of improved hydrologic functions following the restoration of a stream-wetland complex, similar to a Stage 0 restoration, of a headwater valley in the Western Allegheny region of the USA. We monitored hydrologic functions and macroinvertebrate assemblages from stream reaches of a restored and unrestored site over multiple years during the early years following restoration. Reduced bed mobility and increased flow duration indicated improved hydrologic functions from the restored site. Aggregate metrics that capture functional attributes of macroinvertebrate assemblage (i.e., density and biomass) were consistently greater from the restored site. EPT biomass from restored pools was 3–4 × greater than amounts from the unrestored site as a result of consistently greater mayfly abundance. Restored pools also supported a subassemblage of taxa with life history attributes that are aligned with habitat conditions created from improved hydrologic functions. Results from this case study demonstrate the importance of habitat-specific sampling designs that report the absolute abundance of potential biological indicators. Findings from this case study should help guide the development of rapid biological indicators of improved hydrologic functions.

Recharge Estimation Using CMB and Environmental Isotopes in the Verlorenvlei Estuarine System, South Africa and Implications for Groundwater Sustainability in a Semi-Arid Agricultural Region

Groundwater recharge remains one of the most difficult hydrogeological variables to measure accurately, especially for semi-arid environments where the recharge flux is much smaller than in humid conditions. In this study, groundwater recharge was estimated using chloride mass balance (CMB) in the Verlorenvlei catchment, South Africa where the effects of recent severe drought conditions in an already semi-arid environment have impacted both agricultural activity as well as the RAMSAR-listed Verlorenvlei estuarine system. Chloride, 18O and 2H tracers were used to improve understanding of the groundwater flow patterns and allowed the fresh parts of the groundwater system, defined by Ca2+-HCO3− groundwater types, to be separated from those where additional salts were being introduced through groundwater mixing, and thus characterized as Na+-Cl− groundwater types. Recharge rates calculated from CMB in the fresh parts of the system were between 4.2–5.6% and 11.4–15.1% of mean annual precipitation for the headwater valley and mountains of the Krom Antonies and are largely consistent with previous studies. However, much lower recharge rates in the valleys where agriculture is dominant contrasts with previous results, which were higher, since groundwater-mixing zones were not recognised. Although the chloride concentration in precipitation is based on only one year of data between 2015 and 2016, where 2015 had on average 28% less precipitation than 2016, the results provide a snapshot of how the system will respond to increasing drought frequency in the future. The results suggest that low rates of groundwater recharge under dry spell conditions will impact on low flow generations which are required to sustain the Verlorenvlei estuarine lake system. Overall, the study highlights the importance of combining hydrochemical tracers such as bulk chloride and stable isotopes with numerical modelling in data-scarce catchments to fully understand the nature of hydrological resilience.

Impacts of sediment connectivity on Holocene alluvial records across a Mediterranean basin (Guadalentín River, SE-Spain)

Two contiguous alluvial valleys (Maria and Guadalupe sectors) of the upper Guadalentín River (SE Spain) show distinct Holocene alluvial architecture (cut-and-fill, nested and stepped fill terraces), revealing a strong control of valley morphology and bedrock barriers on sediment delivery, storage and preservation. The headwater valley (Maria sector) preserves four morphosedimentary alluvial units corresponding to two palaeogeographic stages. Morphosedimentary unit (MSU1), the most extensive, is a major depositional valley infill dominated by sand and silt texture, showing three stratigraphic sets dated 9000–8500, 5300–4800, 4000–3000 cal BP. This vertically stacked alluvial accumulation occurs upstream of a narrow bedrock channel, that limited sediment connectivity with the downstream Guadalupe sector. Between 3000 and 2350 cal BP, a 15-m deep incision on MSU1 occurred, and subsequent cut-and-fill cycles generated three alluvial terraces dated at 2350–1900 cal BP (MSU2), 1800–700 cal BP (MSU3), and post-400 years (MSU4). In the downstream valley (Guadalupe sector), six flights of alluvial terraces (MSUg1 to MSUg6) were formed by episodic aggradation and quasi-continuous incision, in response to baselevel changes of the lower Guadalentín River during the Holocene. Alluvial chronologies in these two contiguous valleys show out-of-phase sedimentation periods under low connectivity conditions (9–3 ka), and in-phase cut-and-fill cycles after valley re-connection (post-3.0 ka). The late Holocene alluvial activity periods also coincide with morpho-stratigraphic data from the lower Guadalentín, indicating that fluvial connectivity throughout the catchment was only completed in the late Holocene.

Record of coupled hillslope and channel response to Pleistocene erosion and deposition in a sandstone headwater valley, central Pennsylvania

Record of coupled hillslope and channel response to Pleistocene erosion and deposition in a sandstone headwater valley, central Pennsylvania

Creating a More Perennial Problem? Mountaintop Removal Coal Mining Enhances and Sustains Saline Baseflows of Appalachian Watersheds.

Mountaintop removal coal mining (MTM) is a form of surface mining where ridges and mountain tops are removed with explosives to access underlying coal seams. The crushed rock material is subsequently deposited in headwater valley fills (VF). We examined how this added water storage potential affects streamflow using a paired watershed approach consisting of two sets of mined and unmined watersheds in West Virginia. The mined watersheds exported 7-11% more water than the reference watersheds, primarily due to higher and more sustained baseflows. The mined watersheds exported only ~1/3 of their streamflow during storms, while the reference watersheds exported ~2/3 of their annual water yield during runoff events. Mined watersheds with valley fills appear to store precipitation for considerable periods of time and steadily export this alkaline and saline water even during the dry periods of the year. As a result, MTMVFs in a mixed mined/unmined watershed contributed disproportionately to streamflow during baseflow periods (up to >90% of flow). Because MTMVFs have both elevated summer baseflows and continuously high concentrations of total dissolved solids, their regional impact on water quantity and quality will be most extreme and most widespread during low flow periods.

Alluvial terrace development and changing landscape connectivity in the Great Karoo, South Africa. Insights from the Wilgerbosch River catchment, Sneeuberg

Dendritic channel networks in the Wilgerbosch River catchment draining the south side of the Sneeuberg, South Africa, are deeply incised exposing terrace fills of varying thickness and extent. Channel long sections exhibit ‘stepped’ profiles where resistant rock strata cross valley floors but are now partially or completely breached. Using a combination of aerial image analysis, geomorphological mapping, sedimentological investigations (field logging, grain size, and magnetic susceptibility analyses), and geochronology (OSL, 14C), this study demonstrates the patterns and controls on erosion and sedimentation and, to a lesser extent, the age structure of fills in two low-order tributaries (Africanders Kloof and Wilgerbosch Kloof) and several reaches of the higher-order Wilgerbosch River. A conceptual model of terrace development in relation to changing conditions of connectivity is presented. Valley headwaters are dominated by discontinuous palaeochannel and floodout sediments; whilst in second- to fourth-order tributaries, four sedimentologically and stratigraphically distinct terrace fills that exceed the scale and complexity of deposits on the northward side of the Sneeuberg were identified and analysed. The early part of this regional terrace succession highlights the importance of interactions between periglacial and fluvial activity on cut, fill, and pedogenesis around the time of the deglacial period. Terrace development is shown to have been a complex response to reconnection of the channel network with upland colluvial stores resulting in the valleys becoming choked with sediment. This caused a rise in groundwater and formation of extensive calcretised rootmats on valley floors and slopes acting to ‘blanket’ terraces 1 and 2. The thickness and longevity of this blanket is shown to restrict depth of incision in subsequent phases (T3, T4). The deposits in these headwater valleys have, until now, been overlooked as a source of palaeoenvironmental information. This study is the first to demonstrate the role and importance of changing connectivity in 'cut and fill' phases that predate the late eighteenth century European incursion in the Sneeuberg.

Fracture controls on valley persistence: the Cairngorm Granite pluton, Scotland

Valleys are remarkably persistent features in many different tectonic settings, but the reasons for this persistence are rarely explored. Here, we examine the structural controls on valleys in the Cairngorms Mountains, Scotland, part of the passive margin of the eastern North Atlantic. We consider valleys at three scales: straths, glens and headwater valleys. The structural controls on valleys in and around the Cairngorm Granite pluton were examined on satellite and aerial photographs and by field survey. Topographic lineaments, including valleys, show no consistent orientation with joint sets or with sheets of microgranite and pegmatitic granite. In this granite landscape, jointing is not a first-order control on valley development. Instead, glens and headwater valleys align closely to quartz veins and linear alteration zones (LAZs). LAZs are zones of weakness in the granite pluton in which late-stage hydrothermal alteration and hydro-fracturing have greatly reduced rock mass strength and increased permeability. LAZs, which can be kilometres long and >700 m deep, are the dominant controls on the orientation of valleys in the Cairngorms. LAZs formed in the roof zone of the granite intrusion. Although the Cairngorm pluton was unroofed soon after emplacement, the presence of Old Red Sandstone (ORS) outliers in the terrain to the north and east indicates that the lower relief of the sub-ORS basement surface has been lowered by <500 m. Hence, the valley patterns in and around the Cairngorms have persisted through >1 km of vertical erosion and for 400 Myr. This valley persistence is a combined product of regionally low rates of basement exhumation and of the existence of LAZs in the Cairngorm pluton and sub-parallel Caledonide fractures in the surrounding terrain with depths that exceed 1 km.

Deep Impact: Effects of Mountaintop Mining on Surface Topography, Bedrock Structure, and Downstream Waters.

Land use impacts are commonly quantified and compared using 2D maps, limiting the scale of their reported impacts to surface area estimates. Yet, nearly all land use involves disturbances below the land surface. Incorporating this third dimension into our estimates of land use impact is especially important when examining the impacts of mining. Mountaintop mining is the most common form of coal mining in the Central Appalachian ecoregion. Previous estimates suggest that active, reclaimed, or abandoned mountaintop mines cover ∼7% of Central Appalachia. While this is double the areal extent of development in the ecoregion (estimated to occupy <3% of the land area), the impacts are far more extensive than areal estimates alone can convey as the impacts of mines extend 10s to 100s of meters below the current land surface. Here, we provide the first estimates for the total volumetric and topographic disturbance associated with mining in an 11 500 km(2) region of southern West Virginia. We find that the cutting of ridges and filling of valleys has lowered the median slope of mined landscapes in the region by nearly 10 degrees while increasing their average elevation by 3 m as a result of expansive valley filling. We estimate that in southern West Virginia, more than 6.4km(3) of bedrock has been broken apart and deposited into 1544 headwater valley fills. We used NPDES monitoring datatsets available for 91 of these valley fills to explore whether fill characteristics could explain variation in the pH or selenium concentrations reported for streams draining these fills. We found that the volume of overburden in individual valley fills correlates with stream pH and selenium concentration, and suggest that a three-dimensional assessment of mountaintop mining impacts is necessary to predict both the severity and the longevity of the resulting environmental impacts.

Headwater valley response to climate and land use changes during the Little Ice Age in the Massif Central (Yzeron basin, France)

The geomorphological response of valley bottoms in eastern France to climatic fluctuations of the Little Ice Age (LIA) was investigated using sedimentological analysis together with optically stimulated luminescence (OSL) and radiocarbon dating. Diachronic mapping of land use since the beginning of the nineteenth century was also carried out. Since A.D. 1500, the valley bottoms experienced three cycles of aggradation and subsequent incision, each characterized by paired periods of high and low detritic activity. While the impact of human activity on the aggradation of the alluvial plain is observed, the vertical dynamics of the valley bottom deposits seemingly were also linked to the hydroclimatic fluctuations during the LIA. The sensitivity to these fluctuations was increased by human activity at the scale of the basin. Variations of the winter North Atlantic Oscillation (NAO) and solar activity from the last five centuries correlate with wet and cold phases during which valley bottoms accumulated, and dry and warm phases during which the streams incised into the valley floors. This fluvial sensitivity to the meteorological conditions induced temporal variations in sedimentary supply originating from either direct input from remnants of periglacial alluvial sheets or local rocky outcrops and/or from indirect input from the erosion of alluvial and colluvial deposits. These two components, combined with the sheet runoff over the ploughlands, express the complex coupling between hillslopes and valley bottoms in the headwater catchments. This caused a cascade-shaped transit of the sediments characterized by alternating phases of storage and removal.

African wetlands and their seasonal use by wild and domestic herbivores

African wetlands include dambos or headwater valley grasslands in the upper regions of catchments, circular pan grasslands in drainage sumps, linear riverine grasslands in the mid regions of catchments and broad floodplains, swamps and deltas in the lower regions of catchments. Plant zonation on flood frequency and duration gradients occurs in predictable sequences across Africa and this variation combined with increasing retention of soil moisture for plant growth over the dry season with increasing flooding provides critical resource heterogeneity for herbivores. Consequently, wetlands are essential dry-season habitats for wild and domestic herbivore populations across Africa, often providing key resources that buffer herbivore population numbers in the face of severe dry seasons and droughts. Floodplain peripheries and shallow to intermediate-depth floodplains are generally preferred over the early dry season, with deep floodplains and swamps being used by the late dry season and during droughts. Wetland grasslands are generally able to withstand extremely heavy grazing over the dry season without becoming degraded because they are inaccessible to grazers during the period of flooding over the wet season, giving plants a full growing season’s recovery period, as well as by wetland grasses storing a large proportion of their biomass in deep root systems and rhizomes. African wetlands are under siege by upstream damming and by agricultural projects and further/reinforced conservation measures are needed to protect them, such as ecologically designed water releases from dams to maintain flooding regimes and prevention of agriculture on key floodplains for grazers.

Correction of deposit ages for inherited ages of charcoal: implications for sediment dynamics inferred from random sampling of deposits on headwater valley floors

Inherited age is defined herein as the difference between times of carbon fixation in a material and deposition of that material within sediments from which it is eventually sampled in order to estimate deposit age via radiocarbon dating. Inheritance generally leads to over-estimation of the age by an unknown amount and therefore represents unquantified bias and uncertainty that could potentially lead to erroneous inferences. Inherited ages in charcoal are likely to be larger, and therefore detectable relative to analytic error, where forests are dominated by longer-lived trees, material is stored for longer periods upslope, and downstream post-fire delivery of that material is dominated by mass movements, such as in the near-coastal mountains of northwestern North America. Inherited age distribution functions were estimated from radiocarbon dating of 126 charcoal pieces from 14 stream-bank exposures of debris-flow deposits, fluvial fines, and fluvial gravels along a headwater stream in the southern Oregon Coast Range, USA. In the region, these 3 facies are representative of the nearly continuous coalescing fan-fill complexes blanketing valley floors of headwater streams where the dominant transport mechanism shifts from debris-flow to fluvial. Within each depositional unit, and for each charcoal piece within that unit, convolution of the calibrated age distribution with that of the youngest piece yielded an inherited age distribution for the unit. Fits to the normalized sums of inherited age distributions for units of like facies provided estimates of facies-specific inherited age distribution functions. Finally, convolution of these distribution functions with calibrated deposit age distributions yielded corrections to published valley-floor deposit ages and residence time distributions from nearby similar sites. Residence time distributions were inferred from the normalized sums of distributions of ∼30 deposit ages at each of 4 sites: 2 adjacent valley reaches ∼103 m long and within ∼102 m of 2 tributary confluences. Mean inherited ages from the observed distributions are 666, 688, and 1506 yr for debris-flow deposits, fluvial fines, and fluvial gravels, respectively. On average, correction reduced estimates of individual deposit age means by a factor of 0.71 (0.56–0.94) and increased standard deviations by a factor of 6.1 (0.97–43). Across sites, mean residence times decreased by 24.0% and standard deviations by 12.5% on average. Corrected residence time distributions have thicker tails, as indicated by gamma-distribution fits with smaller shape factors, and these changes are significant relative to the bootstrapped 95% confidence limits representing potential error in the sampling for inherited ages. The ratio of the means of sediment age and residence time ranged from 1.03 to 1.80 across sites before correction and 1.21 to 2.18 after correction, where a value of one implies that probability of evacuation from the “reservoir” comprising valley-floor deposits is independent of time since deposition. Corrected values of this ratio therefore indicate that evacuation favors younger deposits at all sites, whereas uncorrected results implied age-independent evacuation from the more downstream valley reach.

Riparian hydraulic gradient and stream‐groundwater exchange dynamics in steep headwater valleys

AbstractPatterns of riparian hydraulic gradients and stream‐groundwater exchange in headwater catchments provide the hydrologic context for important ecological processes. Although the controls are relatively well understood, their dynamics during periods of hydrologic change is not. We investigate riparian hydraulic gradients over three different time scales in two steep, forested, headwater catchments in Oregon (WS01 and WS03) to determine the potential controls of reach‐scale valley slope and cross‐sectional valley geometry. Groundwater and stream stage data collected at high spatial and temporal resolutions over a period encompassing a 1.25 year storm and subsequent seasonal baseflow recession indicate that hydraulic gradients in both riparian aquifers exhibit strong persistence of down‐valley dominance. Responses to rainfall do not support the simple conceptual models of increased riparian hydraulic gradient toward streams. Hydraulic gradient response in WS01 to both the seasonal baseflow recession and the storm suggested the potential for increased stream‐groundwater exchange, but there was less evidence for this in WS03. Results from four constant‐rate tracer injections in each stream showed a high baseline level of exchange overall, and both a slight seasonal increase (WS01) and slight decrease (WS03) in the riparian intrusion of tracer‐labeled stream water as stream discharge receded. These results indicate that steep headwater valley floors host extensive stream water exchange and very little change in the water table gradients over 3 orders of magnitude of stream discharge.

Biotic Drivers of Stream Planform

Traditionally, stream channel planform has been viewed as a function of larger watershed and valley-scale physical variables, including valley slope, the amount of discharge, and sediment size and load. Biotic processes serve a crucial role in transforming channel planform among straight, braided, meandering, and anabranching styles by increasing stream-bank stability and the probability of avulsions, creating stable multithread (anabranching) channels, and affecting sedimentation dynamics. We review the role of riparian vegetation and channel-spanning obstructions—beaver dams and logjams—in altering channel—floodplain dynamics in the southern Rocky Mountains, and we present channel planform scenarios for combinations of vegetation and beaver populations or old-growth forest that control logjam formation. These conceptual models provide understanding of historical planform variability throughout the Holocene and outline the implications for stream restoration or management in broad, low-gradient headwater valleys, which are important for storing sediment, carbon, and nutrients and for supporting a diverse riparian community.

Fluvial connectivity and climate: A comparison of channel pattern and process in two climatically contrasting fluvial sedimentary systems in South Africa

The aim of this research was to investigate the dynamics of valley formation, sediment delivery and channel pattern in two climatically contrasting fluvial sedimentary systems in South Africa. Each system comprised a network of headwater valley fills and floodplains underlain by sedimentary Karoo Supergroup rocks that are intersected by resistant dolerite dykes and sills. The Seekoei River Floodplain and Gordonville valley fill site in the Great Karoo, however, experience less than half the annual precipitation of the Nsonge River Floodplain and Hlatikhulu valley fill in the KwaZulu–Natal Drakensberg Foothills. Furthermore, rainfall is more variable in the Karoo. Despite climatic differences, headwater valley fills were geomorphically similar. In contrast, floodplains in the two regions were vastly different, even when the same downstream control (a resistant dolerite intrusion crossing the drainage line) was considered. Upstream of a dolerite dyke, the Nsonge River is highly sinuous and located in a wide floodplain that has been carved by lateral planation of the underlying bedrock. In comparison, the Seekoei River, located upstream of a dolerite sill, is discontinuous and characterized by floodouts and avulsing distributaries that undergo periods of bedrock incision, followed by infilling.It is likely that this disparity is caused by the inability of infrequent, unsustained flows to develop meanders and, thus, adjust the channel planform to changes in discharge, sediment load and valley slope. Flow variability, thus, exercises a strong control on channel pattern and causes floodouts in headwater settings and the semi-arid Karoo floodplain. As a result, sediment transport in the Seekoei River is likely to be episodic, and net retention of sediment in the semi-arid floodplain is greater than in the sub-humid Nsonge River Floodplain, where sediment depth is limited.

Holocene valley-floor deposition and incision in a small drainage basin in western Colorado, USA

The valley floor of a 33.9 km 2 watershed in western Colorado experienced gradual sedimentation from before ∼ 6765 to ∼ 500 cal yr BP followed by deep incision, renewed aggradation, and secondary incision. In contrast, at least four terraces and widespread cut-and-fill architecture in the valley floor downstream indicate multiple episodes of incision and deposition occurred during the same time interval. The upper valley fill history is atypical compared to other drainages in the Colorado Plateau. One possible reason for these differences is that a bedrock canyon between the upper and lower valley prevented headward erosion from reaching the upper valley fill. Another possibility is that widespread, sand-rich, clay-poor lithologies in the upper drainage limited surface runoff and generally favored alluviation, whereas more clay-rich lithologies in the lower drainage resulted in increased surface runoff and more frequent incision. Twenty-two dates from valley fill charcoal indicate an approximate forest fire recurrence interval of several hundred years, similar to that from other studies in juniper–piñon woodlands. Results show that closely spaced vertical sampling of alluvium in headwater valleys where linkages between hillslope processes and fluvial activity are relatively direct can provide insight about the role of fires in alluvial chronologies of semi-arid watersheds.

Impact of water resource exploitation on the hydrology and water storage in Baiyangdian Lake

AbstractWater resources development and exploitation are critical for a viable and sustainable modern human society. Unfortunately, however, there is a considerable water storage depletion and environmental degradation in especially (semi)‐arid river basins due to the forces of population growth, urbanization, industrialization and intensive agricultural irrigation. Addressing water storage depletion is not only a question of research, but is very much a question of developing appropriate countermeasures to preserve valuable/fragile ecological systems. As one such effort, this study analyzes the hydrology and storage in Baiyangdian Lake as affected by water resources development and exploitation in the Baiyangdian Lake Catchment of Northern China. Three models, WetSpass (Water and Energy Transfer between Soil, Plants and the Atmosphere under quasi‐Steady State), WATBUD (Water Budget) and MODFLOW (USGS three‐dimensional finite‐difference groundwater flow model) were used in combination to simulate the hydrogeologic conditions in the lake catchment for 1956–2008. The model‐calibrated values are in good agreement with the measured values, withR2&gt; 0·8 and RMSE &lt; 10% of measured values. Runoff, the primary source of water for the lake storage, has steadily declined due mainly to multiple dam construction and reservoir impoundments in the headwater valleys and rivers in the catchment. In addition to dwindling runoff, groundwater levels have declined considerably due to over‐abstraction, mainly for agricultural irrigation. Additionally, evaporation or evapotranspiration is increasing in the lake catchment due to rising temperatures. The worsening hydrological conditions, amid the harsh semi‐arid climate, have resulted in considerable depletion of the storage and hydrology of Baiyangdian Lake. Sustainable countermeasures like agricultural water‐saving and infusion of external water (e.g., via by the South–North Water Transfer Project) could be a viable option for preserving not only the hydrology of the lake catchment, but also storage in Baiyangdian Lake. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Roman mining on Exmoor: a geomorphological approach at Anstey's Combe, Dulverton

A survey of valley fills in south-facing combes (headwater valleys) along the south side of the Exmoor massif revealed an anomalously deep infill in one valley. This infill of up to 5 m depth had been gullied revealing a complex stratigraphy. Studies of the stratigraphy, clast orientation and shape suggested several accumulation episodes under different environmental conditions commencing in a periglacial climatic regime. Later units included sandy silts which can be dated using optically stimulated luminescence (OSL) of quartz grains. The OSL dates, indicate that the inter-gravel silts accumulated in two periods, the Romano-British period and the 16th–17th centuries AD. A survey of the very small valley catchment revealed a linear trench of a type associated with early iron mining. Given the anomalously high volume of accumulated sediment from such a small catchment and evidence of mining on the slope above the site, the geomorphic mechanism is almost certainly the downslope transport of mining debris from the slope to the valley floor. This study suggests that the systematic survey of headwater valleys in metalliferous uplands may be one way of locating areas of early mining activity and that such deposits could provide a chronology of working and abandonment.

Evolution of sediment accommodation space in steady state bedrock‐incising valleys subject to episodic aggradation

Steepland valleys subject to debris flows incise bedrock even as episodic deposition typically covers valley bottoms. This paper's hypothesis is that, while continual fluvial processes evacuate deposits, storage of episodic deposition drives valley widening and, thereby, creation of accommodation space for sediment storage on the valley floor. Data from three headwater valleys in the Oregon Coast Range show that valley‐to‐channel width ratios and valley bottom deposit depths are variable, have little systematic variation with respect to contributing area, and are similar on average among sites. A model of valley cross‐section evolution couples soil production, nonlinear diffusion, contrasting rates of channel incision into deposits and bedrock, and stochastic valley bottom deposition. The model reproduces observed flat, deposit‐covered valley bottoms and abrupt transitions to valley sides with oversteepened toe slopes. Simulations address sensitivity of valley morphologies and incision rates to dimensionless numbers, the ratio of instantaneous bedrock and deposit erosion rates (incision number), and the ratio of deposition and evacuation rates (deposition number). For steady state simulations, increasing deposition number by &lt;101 leads to deposit depth and valley bottom width increasing by 101 and 101.5, respectively, and valley bottom incision relative to the instantaneous rate decreasing by 10−3. For incision number increasing by 103, valley capacity (width times toe slope height) relative to mean deposit volume increases by 101.5. Simulations, consistent with field data, imply that steady state valley widths are adjusted to episodic deposition rates and respond more quickly to changes than profile gradients because of contrasting limitations by instantaneous versus long‐term lowering rates.

Sediment storage and evacuation in headwater valleys at the transition between debris-flow and fluvial processes

Sediment from landscape disturbance often enters temporary storage in valleys and evacuates over longer times, which in steeplands are poorly delimited. We hypothesize that, across process transitions (e.g., debris fl ow versus fl uvial transport), distributions of sediment transit times also change. We use fi eld surveys and extensive radiocarbon dating to assess the distribution of transit (residence) times through the proxy measurement of ages of bank deposits in two mainstem reaches of a 2.23 km 2 watershed in the Oregon Coast Range. In the downstream reach, debris fans impound fl uvial deposits; debris-fl ow, fi ne fl uvial, and coarse fl uvial deposits compose nearly equal parts of the valley fi ll; and fl uvial erosion evacuates deposits . Transit times have a sample mean of 1.22 ◊ 10 3 14 C yr and an exponential distribution, indicating uniform probability of evacuation from storage. In the upstream reach, valleyspanning debris jams impound debris-fl ow deposits composing >95% of the valley fi ll, which is routinely scoured by debris fl ows. Transit times have a sample mean of 4.43 ◊ 10 2 14 C yr and, if >100 14 C yr, a power-law distribution, indicating preferential evacuation of younger deposits and retention of older deposits. In both reaches, most sediment has short transit times (<600 14 C yr), but signifi cant volumes remain for millennia. Less than 20% of basin-wide denudation passes through these reservoirs, but the latter are still signifi cant buffers between hillslope disturbance and downstream aquatic habitat, especially for coarse sediment.